In this first reporting period, we have successfully completed all of the tasks in work package 1 and one of the main objectives of work package 2. As part of work package 1, we have designed and built a Brillouin cavity optomechanics system that can operate at milliKelvin temperatures, and used it to measure a GHz frequency high-overtone bulk acoustic wave resonator (HBAR) in the quantum ground state. We overcame a number of significant technical challenges, including the alignment and vibration isolation of the optical system, and investigated the effect of laser heating on the HBAR's mechanical modes. We found that, even for average incident laser powers that are more than sufficient for the eventual goal of microwave-to-optical transduction, our mechanical mode remains below a thermal occupation of one phonon. This work has now been published in Physical Review Research.
As part of work package 2, we have investigated the effect of infrared radiation on superconducting qubits in a systematic way using a novel experimental setup. We were able to control the power and duration of a laser pulse along with its spatial location relative to a transmon superconducting qubit. Using this technique, we were able to study the effect of laser light on the qubit and observe the dynamics of the quasiparticles generated by the high-energy photons, and thereby estimate the amount of time it takes the qubit to recover from the detrimental effects of infrared photons. This not only gave us important information about the possible repetition rates and laser powers that are feasible for operating a quantum transducer, but also more general insight into the behavior of superconducting circuits under high energy radiation, which has important impacts in the field of quantum information science beyond this project.
